1. Field of the Invention
The present invention relates to a power-line carrier method using power transmission lines as communications lines, and more specifically, to a spread spectrum transmission system which transmits data by superimposing spread spectrum modulated signals (hereinafter called as S signals on commercial power lines.
2. Detailed Description of the Related Art
The spread spectrum transmission system is a communication system using the spread spectrum modulation method. Spread spectrum transmission was originally conventionally developed mainly for military use. The communications are encrypted by means of a spread code used as the key. Since the transmitted electric power is spread over a wide bandwidth, spectrum density can be lowered, making it possible to maintain even the existence of the communications secret. Meanwhile, since the spread spectrum transmission system transmits data by spreading it over a wide bandwidth, it is possible to transmit data even if the signal strength of the signal electric power is, in places, weaker than the noise of the transmission line. In recent years, based on these observations, a commercial spread spectrum transmission system has been developed using commercial electric power lines as signal transmission lines, for such applications as the remote control of machine tools in factories, where the generation of noise cannot be avoided.
Referring to FIG. 3, a conventional spread spectrum transmission system 100, employed for such applications as the remote control of machine tools inside a factory, uses the same commercial electric power line 101 wired from outside the building to the inside of the factory via a breaker 102. A blocking filter 103 connected downstream of the breaker 102 prevents the infiltration of external electrical noise via the breaker 102 from the transmission lines of an electric power company. Additionally, the blocking filter 103 is installed so as to prevent the external leakage of SS signals flowing along the commercial electric power lines 101a and 101b inside the factory, to be described later.
A plurality of commercial electric power lines 101a and 101b branch to each process within the factory, including assembly lines, molding lines, and the like, and to plug sockets 104, 104 . . . The plug sockes 104 are installed at convenient locations along the power lines 101a/101b to provide power to load devices such as machine tools and electrical apparatuses.
For example, the socket plugs of a press molding device 105 and an illuminating device 106 are connected to the plug sockets 104, 104 . . . of the commercial electric power line 101a. The socket plugs of assembling devices 107 and 108 are connected to the plug sockets 104, 104 . . . of the commercial electric power line 101b. Commercial alternating current power is thus supplied to each load device.
The socket plugs of a plurality of spread spectrum communication terminal apparatuses 109a, 109b, 109c, and 109d are plugged into freely selected remaining plug sockets 104, 104 . . . of the commercial electric power lines 101a and 101b. These spread spectrum communication terminal apparatuses 109 are each equipped with an input-output portion which inputs and outputs communication signals, and a spread spectrum modem apparatus (hereinafter called as S modem) which modulates and demodulates between communication signals and SS signals. Each S modem superimposes SS communication signals onto the commercial electric power line 101 to which its socket plug is connected. In addition each S modem outputs SS signals flowing on the commercial electric power line 101 as communication signals, to or from the input-output portion.
For example, the spread spectrum communication terminal apparatus 109a connected to the commercial electric power line 101a is installed adjacent the press molding device 105. The spread spectrum communication terminal apparatus 109a receives and decodes communication signals indicating operational information of the press molding device 105. Information to and from the spread spectrum communication terminal apparatus 109a passes along communication cables shown as dotted lines in the drawing. This operational information is superimposed on the commercial electric power line 101a as SS signals, demodulated to communication signals by the spread spectrum communication terminal apparatus 109b similarly connected to the commercial electric power line 101a, and output to a personal computer (hereinafter called as PC 110 connected to the input-output portion of the spread spectrum communication terminal apparatus 109b. Additionally, the PC 110 can output control information for controlling the press mold 105. Communication signals representing the control information are sent to the press molding device 105 via the commercial electric power line 101a, by flow in the opposite direction to that described above.
In the same way, communication signals can be transmitted between assembling devices 107 and 108 and a PC 111 installed in a freely chosen location, by means of the spread spectrum communication terminal apparatus 109c connected to the assembling devices 107 and 108. These communication signals travel along the communication cables shown by the dotted lines, and a spread spectrum communication terminal apparatus 109d connected to the PC 111. Moreover, if PC 110 and PC 111 are connected to another communication network 112, all machine tools in the factory can be centrally managed from a host computer, not shown in the drawing, connected to the communication network 112 (noted as a local area network LAN).
Thus, with the spread spectrum transmission system 100 using the commercial electric power line 101, it is possible to connect an SS modem, which modulates and demodulates SS signals between data terminal apparatuses (control-receiving apparatuses), using a commercial electric power line 101, and transmit data between the spread spectrum communication terminal apparatuses 109 using this commercial electric power line 101 as a signal transmission line. Consequently, in an environment having ordinary power supply equipment, it is possible to easily transmit data between terminal apparatuses without preparing a new communication line.
With the above-mentioned conventional spread spectrum transmission system 100, blocking filter 103 is required after the breaker 102 which is the lead-in for the commercial electric power line 101 from outside the facility, in order to prevent the infiltration of external noise, and to prevent external leakage of SS signals superimposed over commercial power lines inside the factory.
However, in order to serially interpose the blocking filter 103 in the commercial electric power line 101 through which all the load devices in the factory are supplied with electric power, blocking filter 103 must be an expensive, high-capacity blocking filter.
Additionally, the spread spectrum modulation method is not easily impacted by noise because it diffuses noise by back-diffusion during demodulation. However, machine tools inside a factory are prone to generate high levels of noise which can produce an outflow of electrical noise to the commercial electric power line 101. Consequently, in order to conduct stable communications using a commercial electric power line 101 as a communication line, a noise filter is required on each load device, increasing the overall cost of the system.
An object of the present invention is to provide an inexpensive spread spectrum transmission system which prevents the infiltration of external noise, and prevents the external leakage of SS signals.
Additionally, another object of the present invention is to provide a spread spectrum transmission system which allows stable communication by means of SS signals, without installing filters on load devices receiving commercial alternating current electric power.
In order to solve the above-mentioned problems, a first aspect of the present invention is a spread spectrum transmission system comprising: a commercial electric power line, equipped with a plug socket, and supplying commercial alternating current electric power to a load device, an electrical plug of which is connected to the plug socket; an alternating current power supply line for communications one end of which being equipped with an electrical plug plugged into the plug socket, and connected to the commercial electric power line; a plug socket for terminal connection, being connected to the alternating current power supply lines for communications at at least one position of the alternating current power supply lines for communications; a blocking filter interposed on the alternating current power supply lines for communications between the electrical plug and the plug socket for terminal connection; and a spread spectrum communication terminal apparatus, having a communication signal input-output portion and a spread spectrum modem portion, and which, with respect to the alternating current power supply line for communications connected by plugging the plug for terminal connection into the plug socket for terminal connection, superimposes a modulated communication signal as a spread spectrum modulated signal thereon, or isolates a spread spectrum modulated signal therefrom and demodulates it into a communication signal.
The load devices are connected to the commercial electric power line, the electrical plugs of the load devices are plugged into the plug sockets of the electric power line to supply commercial alternating current electric power thereto from the commercial electric power line. Additionally, the spread spectrum communication terminal apparatus is connected to the alternating current power supply line for communications, which is connected to the commercial electric power line via the blocking filter, and spread spectrum modulated signals are superimposed on the alternating current power supply line for communications, using the alternating current power supply line for communications as a communications line.
External noise transmitted from the commercial electric power line, and noise generated by the load devices are prevented from infiltrating into the alternating current power supply line for communication by the blocking filter interposed in the alternating current power supply line for communication, and SS signals superimposed on the alternating current power supply line for communication are cut off by the blocking filter, and are not leaked outside the facility.
Since only devices with small loads, such as spread spectrum communication terminal apparatuses, are connected to the alternating current power supply line for communication, only small amounts of current flow thereon, and a high-capacity blocking filter is not required.
Additionally, since the blocking filter prevents noise generated by load devices from being transmitted to the spread spectrum communication terminal apparatuses, blocking filters on each load device can be omitted.
A second aspect of the present invention is the spread spectrum transmission system according to the first aspect, further characterized in that the plug for terminal connection is formed in the same shape as the plug socket, and is freely connected to the plug sockets installed on the commercial electric power line.
It is possible to plug the electrical plug of a conventional spread spectrum communication terminal apparatus, which had been plugged into a plug socket of a commercial electric power line, into the plug socket for terminal connection of the alternating current power supply line for communication as a plug for terminal connection. Consequently, it is possible to use a conventional spread spectrum communication terminal apparatus as the spread spectrum communication terminal apparatus connected to an alternating current power supply line for communication according to the first aspect, without any modification.
In an application without the problems of noise infiltration or leakage of SS signals, the spread spectrum communication terminal apparatus can be connected to the commercial electric power line to which other load devices are connected without any modification.
A third aspect of the present invention is the spread spectrum transmission system according to the second aspect, further characterized in that the alternating current power supply line for communication is freely separable at one side of the blocking filter, by the plug socket being installed on one side of the blocking filter and an electrical plug installed on one end of an alternating current power supply line for communication connected with the plug socket for terminal connection.
The alternating current power supply line for communication separated at the blocking filter has the same construction as an existing power supply cable with attached plug socket which is connected to a commercial electric power line, and this existing power supply cable can be used as-is as a portion of the alternating current power supply line for communication.
The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.